Polytetrafluoroethylene (PTFE) is a useful polymeric material that has found application in the construction of many devices which utilize its properties of inertness to chemical attack, hydrophobicity and biocompatibility. For example, it is used as a coating to protect chemical tanks which are used to hold corrosive chemicals; it is used as a moisture barrier in industrial applications to protect sensitive scientific instrumentation and also in rainwear garments; it is also used in biomedical applications for implantable devices such as vascular grafts and surgical patches.
A particularly useful form of PTFE is microporous PTFE. There are a number of ways to make microporous PTFE. One method is by an expansion process such as that described in U.S. Pat. No. 3,953,566. This patent describes a means of producing a microporous structure characterized by nodes connected by fibrils made by rapid expansion of PTFE. Another method is by a replication process such as that described in U.S. Pat. No. 3,497,256 which describes a means of sintering PTFE particles onto a fibrous substrate. Alternatively, microporous PTFE can be made by the removal of fugitive materials after extrusion as described in claim 1 of U.S. Pat. No. 4,613,544.
Microporous PTFE membranes are useful in a number of applications as either filters or venting barriers. These membranes are particularly useful materials in the design and construction of medical, pharmaceutical and other devices because they are hydrophobic materials which function as a barrier to liquid water and aqueous solutions or mixtures but because the membranes are microporous, they allow the flow of air or other gases and, if the pores are sufficiently small, they can be made to exclude microorganisms and other microscopic contaminants. These membranes and the devices constructed with these materials are commonly used in medical or pharmaceutical applications which require sterilization. There are a number of methods of sterilization which are used by manufacturers of such devices, including sterilization by steam, ethylene oxide gas, or ionizing radiation. The ionizing radiation can be of several types, such as, gamma irradiation or high energyelectron beam bombardment, among others. There are significant economic and industrial hygiene advantages to using either gamma irradiation or electronbeam bombardment. However, while these methods of sterilization are more convenient, they also can cause more damage to the materials used in the device than the other means of sterilization. Although microporous PTFE exhibits extraordinary resistance to chemical attack from a wide variety of chemical species, it has only a very limited resistance to ionizing radiation. Microporous PTFE is particularly sensitive to degradation by ionizing radiation in comparison to other polymeric materials; it is usually rated as the polymeric material most susceptible to degradation by ionizing radiation. There is a need for a method by which microporous PTFE material can be used in applications which require or prefer the use of radiation sterilization and subsequent pressurization.
Specifically, the use of microporous PTFE in numerous applications is greatly limited by its marked susceptibility to the degradation of its physical properties such as ultimate tensile strength and ultimate elongation as the result of exposure to ionizing radiation. This decrease in physical properties is observed at relatively low levels of radiation; significant decreases are observed at radiation exposures of less than 0.5 Megarad. This radiation sensitivity imposes a serious limitation on the use of microporous PTFE in applications in which it is necessary for the material to withstand even relatively low levels of radiation. Gamma sterilization is generally accomplished by exposing the devices to be sterilized to 2.5 Megarad or more. An example of an application where radiation is typically encountered is medical or pharmaceutical devices which can be conveniently sterilized with ionizing radiation. It is necessary to use a dose of ionizing radiation sufficiently large to ensure that the article is completely sterile. This is commonly achieved by placing the articles to be sterilized in a shielded room with a relatively powerful irradiation source, such as a cobalt-60 source. The sources are usually of sufficient strength that the sterilization procedure can be accomplished in 60 to 120 minutes. The sterilization is normally performed after the microporous PTFE laminate has been incorporated into the device after the completion of all manufacturing steps. Another example is in space applications where the microporous PTFE is exposed to low doses of ionizing radiation for long periods of time.
This susceptibility to damage by ionizing radiation is particularly acute in applications which use thin sheets of PTFE and/or microporous or expanded PTFE. In these applications because of the limited amount of polymer which is present [as compared to devices which use thick pieces (e.g., slabs or rods) of full density PTFE, the decrease in the physical properties is often prohibitively detrimental. For many years this has prevented the use of microporous PTFE materials in applications which are sterilized with ionizing radiation.
This patent describes composites and devices which permit the use of thin sheets of microporous PTFE in those applications which require both sterilization with ionizing radiation and the ability to withstand sustained pressurization without rupturing or failing. Said composites and devices achieve greater than a one hundred fold increase in the useful lifetime with respect to existing laminates and devices.